Fluoroantimonic acid

Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony penta­fluoride, containing various cations and anions (the simplest being H
and SbF
). This substance is a superacid that can be over a billion times stronger than 100% sulfuric acid in terms of its protonating ability measured by Hammett function. It even protonates some hydro­carbons to afford pentacoordinate carbo­cations (carbonium ions).[1] Fluoro­antimonic acid is corrosive. For example, it cannot be contained directly in glass carboys, as it attacks glass,[2] but can be stored in containers lined with PTFE (Teflon) or PFA polymers.

Fluoroantimonic acid
Fluoroantimonic acid-3D-balls.png
IUPAC name
Fluoroantimonic acid
Systematic IUPAC name
Hexafluoroantimonic acid
Other names
  • Fluoroantimonic(V) acid
  • Hydrogen fluoroantimonate
  • Fluoronium Fluoroantimonate
  • Fluoronium Hexafluoroantimonate
3D model (JSmol)
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EC Number
  • 241-023-8
  • InChI=1S/FH2.6FH.Sb/h1H2;6*1H;/q+1;;;;;;;+5/p-6 ☒N
  • [FH2+].F[Sb-](F)(F)(F)(F)F
Appearance Colorless liquid
Density 2.885 g/cm3
Reacts explosively
Solubility SO2ClF, SO2
Occupational safety and health (OHS/OSH):
Main hazards
Extremely corrosive, toxic, violent hydrolysis, oxidizer.
GHS labelling:
GHS05: CorrosiveGHS06: ToxicGHS07: Exclamation markGHS03: OxidizingGHS09: Environmental hazardGHS08: Health hazard
H300+H310+H330, H314, H411
P260, P264, P273, P280, P284, P301+P310
NFPA 704 (fire diamond)
Related compounds
Related acids
Antimony pentafluoride
Hydrogen fluoride
Magic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Chemical compositionEdit

Fluoroantimonic acid is formed by combining hydrogen fluoride and antimony pentafluoride:

SbF5 + 2 HF ⇌ SbF
+ H2F+

The speciation (i.e., the inventory of components) of "fluoroantimonic acid" is complex. Spectroscopic measurements show that fluoroantimonic acid consists of a mixture of HF-solvated protons, [(HF)nH]+ (such as H3F+2), and SbF5-adducts of fluoride, [(SbF5)nF] (such as Sb4F21). Thus, the formula "[H2F]+[SbF6]" is a convenient but oversimplified approximation of the true composition.[3] Nevertheless, the extreme acidity of this mixture is evident from the exceptionally poor proton-accepting ability of the species present in solution. Hydrogen fluoride, a weak acid in aqueous solution that is normally not thought to have any appreciable Brønsted basicity at all, is in fact the strongest Brønsted base in the mixture, protonating to H2F+ in the same way water protonates to H3O+ in aqueous acid. As a result, the acid is often said to contain "naked protons", though the "free" protons are, in fact, always bonded to hydrogen fluoride molecules.[4] It is the fluoronium ion that accounts for fluoroantimonic acid's extreme acidity. The protons easily migrate through the solution, moving from H2F+ to HF, when present, by the Grotthuss mechanism.

Two related products have been crystallized from HF-SbF5 mixtures, and both have been analyzed by single crystal X-ray crystallography. These salts have the formulas [H
and [H
. In both salts, the anion is Sb
.[5] As mentioned above, SbF
is weakly basic; the larger anion Sb
is expected to be a still weaker base.


Fluoroantimonic acid is the strongest superacid based on the measured value of its Hammett acidity function (H0), which has been determined for various ratios of HF:SbF5. The H0 of HF is −15. A solution of HF containing 1 mol % of SbF5 is −20. The H0 is −21 for 10 mol%. For > 50 mol % SbF5, the H0 is between −21 and −23. The lowest attained H0 is about -28.[6][7][8] The following H0 values show that fluoroantimonic acid is stronger than other superacids.[9] Increased acidity is indicated by smaller (in this case, more negative) values of H0.

Of the above, only the carborane acids, whose H0 could not be directly determined due to their high melting points, may be stronger acids than fluoroantimonic acid.[9][10]

The H0 value measures the protonating ability of the bulk, liquid acid, and this value has been directly determined or estimated for various compositions of the mixture. The pKa on the other hand, measures the equilibrium of proton dissociation of a discrete chemical species when dissolved in a particular solvent. Since fluoroantimonic acid is not a single chemical species, its pKa value is not well-defined.[citation needed]

The gas-phase acidity (GPA) of individual species present in the mixture have been calculated using density functional theory methods.[3] (Solution-phase pKas of these species can, in principle, be estimated by taking into account solvation energies, but do not appear to be reported in the literature as of 2019.) For example, the ion-pair [H2F]+·SbF
was estimated to have a GPA of 254 kcal/mol. For comparison, the commonly encountered superacid triflic acid, TfOH, is a substantially weaker acid by this measure, with a GPA of 299 kcal/mol.[11] However, certain carborane superacids have GPAs lower than that of [H2F]+·SbF
. For example, H(CHB11Cl11) has an experimentally determined GPA of 241 kcal/mol.[12]


Fluoroantimonic acid is so reactive that it is challenging to identify media with which it is unreactive. Materials compatible with fluoroantimonic acid as a solvent include SO2ClF, and sulfur dioxide; some chlorofluorocarbons have also been used. Containers for HF/SbF5 are made of PTFE.

Fluoroantimonic acid decomposes when heated, generating free hydrogen fluoride gas and liquid antimony pentafluoride at a temperature of 40 °C.[13]

As a superacid, fluoroantimonic acid protonates nearly all organic compounds, often causing dehydrogenation, or dehydration. In 1967, Bickel and Hogeveen showed that 2HF·SbF5 will remove H2 from isobutane and methane from neopentane to form carbenium ions:[14][15]

(CH3)3CH + H+ → (CH3)3C+ + H2
(CH3)4C + H+ → (CH3)3C+ + CH4

It is also used in the synthesis of tetraxenonogold complexes.[16]


HF/SbF5 is a highly corrosive substance that reacts violently with water.[10] Heating fluoroantimonic acid is dangerous as well, as it decomposes into toxic hydrogen fluoride gas.[2] The main method of containment involves storage in a PTFE container as it dissolves glass and many other materials.[2]

See alsoEdit


  1. ^ Olah, G. A. (2001). A Life of Magic Chemistry: Autobiographical Reflections of a Nobel Prize Winner. John Wiley and Sons. pp. 100–101. ISBN 978-0-471-15743-4.
  2. ^ a b c Anne Marie Helmenstine, Ph.D. "What Is the World's Strongest Superacid?". ThoughtCo. Retrieved April 13, 2022.
  3. ^ a b Esteves, Pierre M.; Ramírez-Solís, Alejandro; Mota, Claudio J. A. (March 2002). "The Nature of Superacid Electrophilic Species in HF/SbF5: A Density Functional Theory Study". Journal of the American Chemical Society. 124 (11): 2672–2677. doi:10.1021/ja011151k. ISSN 0002-7863. PMID 11890818.
  4. ^ Klein, Michael L. (October 25, 2000). "Getting the Jump on Superacids" (PDF). Pittsburgh Supercomputing Center (PSC). Archived from the original (PDF) on May 31, 2012. Retrieved 2012-04-15.
  5. ^ Mootz, Dietrich; Bartmann, Klemens (March 1988). "The Fluoronium Ions H2F+ and H
    : Characterization by Crystal Structure Analysis". Angewandte Chemie International Edition. 27 (3): 391–392. doi:10.1002/anie.198803911.
  6. ^ Superacid chemistry. Olah, George A. (George Andrew), 1927–2017., Olah, George A. (George Andrew), 1927–2017. (2nd ed.). Hoboken, N.J.: Wiley. 2009. ISBN 9780470421543. OCLC 391334955.{{cite book}}: CS1 maint: others (link)
  7. ^ Olah, G. A. (2005). "Crossing Conventional Boundaries in Half a Century of Research". Journal of Organic Chemistry. 70 (7): 2413–2429. doi:10.1021/jo040285o. PMID 15787527.
  8. ^ It has been estimated that HF-SbF5 mixtures have H0 values as low as –28. H0 values down to –27 have been estimated for FSO3H-SbF5 at 90% SbF5, but other measurements do not support H0 values lower than about –24 for either magic acid or fluoroantimonic acid.
  9. ^ a b Gillespie, R. J.; Peel, T. E. (1973-08-01). "Hammett acidity function for some superacid systems. II. Systems sulfuric acid-[fsa], potassium fluorosulfate-[fsa], [fsa]-sulfur trioxide, [fsa]-arsenic pentafluoride, [sfa]-antimony pentafluoride and [fsa]-antimony pentafluoride-sulfur trioxide". Journal of the American Chemical Society. 95 (16): 5173–5178. doi:10.1021/ja00797a013. ISSN 0002-7863.
  10. ^ a b Olah, G. A.; Prakash, G. K. Surya; Wang, Qi; Li, Xing-ya (15 April 2001). "Hydrogen Fluoride–Antimony(V) Fluoride". Encyclopedia of Reagents for Organic Synthesis. New York: John Wiley and Sons. doi:10.1002/047084289X.rh037m. ISBN 9780470842898.
  11. ^ Koppel, Ilmar A.; Burk, Peeter; Koppel, Ivar; Leito, Ivo; Sonoda, Takaaki; Mishima, Masaaki (May 2000). "Gas-Phase Acidities of Some Neutral Brønsted Superacids: A DFT and ab Initio Study". Journal of the American Chemical Society. 122 (21): 5114–5124. doi:10.1021/ja0000753. ISSN 0002-7863.
  12. ^ Meyer, Matthew M.; Wang, Xue-bin; Reed, Christopher A.; Wang, Lai-Sheng; Kass, Steven R. (2009-12-23). "Investigating the Weak to Evaluate the Strong: An Experimental Determination of the Electron Binding Energy of Carborane Anions and the Gas phase Acidity of Carborane Acids". Journal of the American Chemical Society. 131 (50): 18050–18051. doi:10.1021/ja908964h. ISSN 0002-7863. PMID 19950932. S2CID 30532320.
  13. ^ Oelderik, Jan (December 1966). "Werkwijze ter bereiding van halogeenverbindingen van vijfwaardig antimoon". Netherlands Patent Application. NL 6508096 A.
  14. ^ Bickel, A. F.; Gaasbeek, C. J.; Hogeveen, H.; Oelderik, J. M.; Platteeuw, J. C. (1967). "Chemistry and spectroscopy in strongly acidic solutions: reversible reaction between aliphatic carbonium ions and hydrogen". Chemical Communications. 1967 (13): 634–635. doi:10.1039/C19670000634.
  15. ^ Hogeveen, H.; Bickel, A. F. (1967). "Chemistry and spectroscopy in strongly acidic solutions: electrophilic substitution at alkane-carbon by protons". Chemical Communications. 1967 (13): 635–636. doi:10.1039/C19670000635.
  16. ^ Konrad Seppelt, Stefan Seidel; Seppelt, K (2000-10-06). "Xenon as a Complex Ligand: The Tetraxenonogold(II) Cation in AuXe2+
    ". Science. 290 (5489): 117–118. Bibcode:2000Sci...290..117S. doi:10.1126/science.290.5489.117. PMID 11021792.